VITRIFIED BRICK AS MATERIAL FOR COUNTRY ROADS. 5 
COLOR. 
The color is a valuable guide in inspecting brick from the same 
plant, but it is of little importance when the brick to be compared 
are from different factories. For brick manufactured from a par- 
ticular raw material the color indicates, in a measure, the tempera- 
ture to which they have been subjected, provided they have been 
burned under identical conditions. Ordinarily, the darker the color 
the higher the temperatuie and, presumably, the better the brick. 
The surface color of brick may be very misleading, however, and the 
color of the interioi should be used in making comparisons. 
SPECIFIC GRAVITY. 
The specific gravity of paving brick was formerly considered of 
importance in judging their fitness for use in pavements. It has 
since been generally conceded, however, that a knowledge of the 
specific gravity is of comparatively little value. The specific gravity 
of shale brick, is ordinarily between 2.20 and 2.40, and of fire-clay 
brick between 2.10 and 2.25. 
ABSORPTION. 
The absorptive power of brick, like their color, is a matter of very 
slight importance, except for comparing specimens manufactured 
under identical conditions. It is true that the porosity of the brick 
increases with the power of absorption, but it is very doubtful if any 
paving brick possessing an objectionably high absorptive power could 
pass even a very casual inspection. In other words, a high degree of 
poiosity always manifests itself in other ways more clearly than in 
the ability of the brick to absorb water. 
CRUSHING STRENGTH. 
The crushing strength of good paving brick varies from 10,000 
pounds to 20,000 pounds per square inch when the load is applied 
uniformly over the entire top surface of the test specimen, and may be 
much greater if the area over which the load is applied is less than 
that of the top surface. Since paving brick in use are seldom required 
to withstand a pressure of more than about 2,000 pounds per square 
inch and since inferior brick may possess relatively very high resist- 
ance to crushing, a knowledge of the crushing strength is clearly of 
little value in compaiing the relative excellence of different makes of 
brick. It is, therefore, usually considered unnecessary to specify a 
definite requirement as to the crushing strength of paving brick. 
TESTING THE BRICK. 
Definite methods of testing paving brick have been in general use 
for only a comparatively few years and have only recently under- 
gone a pronounced change. The object of all tests is to determine 
